An important goal in designing a wireless communication system is to maximize system capacity, that is, to maximize a number of users that may simultaneously be served by the communication system. One way of increasing system capacity is to lower the transmit power allocated to each user. By lowering the allocated transmit power, interference for all users is lowered, which allows for the addition of new users to the system.
One way to lower the transmit power allocated to each user, or subscriber unit, is to increase the efficiency of the wireless link, or communication channel, between the subscriber unit and the base station serving that user. One method of increasing the efficiency of the link is to broadcast information to a target subscriber unit using a transmit antenna diversity system. A transmit antenna diversity system allows the transmitted signal to be beamformed so that a narrower, more focused beam is transmitted to the user. Antenna beamforming allows for a base station to lower the transmit power allocated to the signal, since the allocated power is less widely dispersed. Antenna beamforming also reduces multipath fading of the transmitted signal and interference with non-targeted users since the beam is more narrowly focused.
One method of antenna beamforming involves two-antenna diversity. Two-antenna diversity uses an antenna array consisting of two antennas, or array elements, to transmit a signal and then applies an optimization technique to improve the quality of the transmitted signal over the performance that would be afforded by the use of a single antenna. One of the simplest forms of two-antenna diversity is two-antenna selection transmit diversity (STD). As its name implies, this method involves selecting one of two antennas as the antenna that will be utilized as the transmitter for a particular communication. A typical method of selecting an antenna involves choosing the antenna that has the highest received power with respect to training, synchronization, or data communications exchanged with the target subscriber unit.
Another method of antenna beamforming involves separately weighting the signal transmitted by each element of the antenna array. If the elements of the antenna array are weighted and phased appropriately, the signals broadcast from these elements will add constructively at a receiver of the target subscriber unit. However, two conditions must be met before an optimal weighting can be applied to the transmit array. First, the channel between each of the array elements and the subscriber of interest must be known. Secondly, it must be possible to compute the signal-to-noise ratio of the subscriber unit of interest.
Previous work has been performed on optimizing the weightings determined at an antenna array for each of multiple subscribers units. Current methods for weighting the coefficients (hereinafter referred to as “TxAA”) are optimal only if the interference environment of the target subscriber unit is dominated by inter-cell interference (optimal in the sense of maximizing the signal-to-noise ratio at the subscriber unit for a given level of transmit power). However, in many applications, especially data applications, the target subscriber unit is close to the base station, where self-interference is the dominant source of interference. In these conditions, the existing method for optimizing the transmitter weighting coefficients can be highly non-optimal.
In addition, current methods for weighting the coefficients do not account for the self-interference introduced by multipath delay. In fact, the current method is optimal only if one of the following two conditions hold; (i) the propagation channel has only a single path, or (ii) the ratio of intra-cell interference power to inter-cell interference power is zero. The latter condition can never be met precisely, but may be an acceptable approximation in some circumstances. In the absence of conditions (i) or (ii), situations arise where the current methods for weighting the coefficients perform worse than a much simpler selection transmit diversity (STD) weighting system.
As a result, current methods for weighting the coefficients are less than optimal for many operating environments. Therefore, an antenna array beamfoming technique is needed that optimizes performance of an antenna array in a high multipath fading environment or in environments of significant self-interference or where intra-cell interference dominates inter-cell interference.